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Abstract

This work presents the development of an optical setup for
quantitative, high-temporal resolution line-of-sight extinction
imaging in harsh optical environments. The application specifically
targets measurements of automotive fuel sprays at high ambient
temperature and pressure conditions where time scales are short and
perceived attenuation by refractive index gradients along the optical
path (i.e., beam steering) can be significant. The illumination and
collection optics are optimized to abate beam steering, and the design
criteria are supported by well-established theoretical relationships.
The general effects of refractive steering are explained conceptually
using simple ray tracing. Three isolated scenarios are analyzed to
establish the lighting characteristics required to render the observed
radiant flux unaffected by the steering effect. These criteria are
used to optimize light throughput in the optical system, enabling
minimal exposure times and high-temporal resolution capabilities. The
setup uses a customized engineered diffuser to transmit a constant
radiance within a limited angular range such that radiant intensity is
maximized while fulfilling the lighting criteria for optimal
beam-steering suppression. Methods for complete characterization of
the optical system are detailed. Measurements of the liquid–vapor
boundary and the soot volume fraction in an automotive spray are
presented to demonstrate the resulting improved contrast and reduced
uncertainty. The current optical setup reduces attenuation caused by
refractive index gradients by an order of magnitude compared to
previous high-temporal resolution setups.

Figures (13)

Illustrations of acceptance cones collecting light to a specific
pixel on an imaging sensor. Chief and marginal rays are traced
through (a) parallel-faced, (b) non-parallel-faced, and
(c) non-planar-faced refracting media. The solid lines trace the
refracted marginal rays, the dashed lines the un-refracted
marginal rays, and the dotted line shows the refracted chief ray.
The plot to the right illustrates the angular distribution of
radiant intensity, and the shaded area represents the radiant flux
received by the pixel.

Angular distribution in image plane of the Ghandhi and Heim and
Thomson et al. optical setups as calculated
with non-sequential ray tracing using commercial software
ZEMAX. Y-axis values are not representative, as the
results have been scaled for comparison.

Measured acceptance angle ω and steepest angle α (bottom) at various points in the image plane
with a 50 mm f/1.2 lens objective equipped with a two-diopter
close-up lens. The corresponding angles are illustrated in the top
figure and indicated in the insets on the plot. The measurements
used a 1% decrease in pixel count threshold.

Illustration demonstrating how beam steering can potentially induce
an apparent optical thickness. Beams being steered out induce a
positive optical thickness, while beams being steered in induce a
negative optical thickness.

(Top) Specific optical arrangement used to generate the input to
the large engineered diffuser. Also shown is the radiance
measurement technique used to characterize the lighting. (Middle)
Measured angular intensity distribution at three locations across
the imaging plane with collimated input to the diffuser. (Bottom)
Measured angular intensity distribution at three locations across
the imaging plane with a slightly focused input to the
diffuser.

Instantaneous τ images of DBIEI-LL measurement from a 90 μm
orifice injector into an ambient with Tamb=900K and ρamb=22.8kg/m3 using (a) modified Ghandhi and Heim setup with a
450 nm LED and 82 kHz frame rate, (b) current setup with a 623-nm
LED, 15-deg diffuser, and 150 kHz frame rate, and (c) same setup
with focusing to the diffuser.

Instantaneous KL images of DBIEI-soot measurement from 90 μm
orifice injector into an ambient with Tamb=900K and ρamb=22.8kg/m3 using, (a) modified Ghandhi and Heim setup with
623-nm LED and 41 kHz frame rate, (b) current setup with 623-nm
LED, 15-deg diffuser, and 45 kHz frame rate and (c) same setup
with focusing to the diffuser.

Illustration of irradiance quantified through the focal point of
one pixel from an extended light source with constant radiance
subtended by the solid angle, Ω, of the camera objective. Also illustrated is
the radiant exchange between an infinitesimal area in the focal
point and an infinitesimal area on the source.